Extending IP Addresses 83 255.255.255.240 = /28 255.255.255.248 = /29 255.255.255.252 = /30 Notice that the CIDR list starts at a minimum of /8 and can’t go higher than /30. This is because the mask must at least be a Class A default, and you must leave two hosts at a minimum. Let’s now take a look at how Cisco handles CIDR. Cisco and CIDR Cisco has not always followed the CIDR standard. Take a look at the way a Cisco 2500 series router asks you to put the subnet mask in the configura- tion when using the Setup mode: Configuring interface Ethernet0: Is this interface in use? [yes]: return Configure IP on this interface? [yes]: return IP address for this interface: 1.1.1.1 Number of bits in subnet field [0]: 8 Class A network is 1.0.0.0, 8 subnet bits; mask is /16 Notice that the router asks for the number of bits used only for subnet- ting, which does not include the default mask. When dealing with these ques- tions, remember that your answers involve the number of bits used for creating subnets, not the number of bits in the subnet mask. The industry standard is that you count all bits used in the subnet mask and then display that number as a CIDR, for example, /25 is 25 bits. The newer IOS that runs on Cisco routers, however, runs a Setup script that no longer asks you to enter the number of bits used only for subnetting. Here is an example of a new 1700 series router in Setup mode: Configure IP on this interface? [no]: y IP address for this interface: 1.1.1.1 Subnet mask for this interface [255.0.0.0]: 255.255.0.0 Class A network is 1.0.0.0, 16 subnet bits; mask is /16 Notice that the Setup mode asks you to enter the subnet mask address. It then displays the mask using the slash notation format. Much better. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 84 Chapter 3 IP Addressing Route Summarization In the “Design Considerations with VLSM” section, we briefly mentioned the concept of route summarization. So, what is it, and why do we need it? On very large networks, there may be hundreds or even thousands of indi- vidual networks and subnetworks being advertised. All these routes can be very taxing on a router’s memory and processor. In many cases, the router doesn’t even need specific routes to each and every subnet (e.g., 172.16.1.0/24). It would be just as happy if it knew how to get to the major network (e.g., 172.16.0.0/16) and let another router take it from there. A router’s ability to take a group of subnetworks and summa- rize them as one network (i.e., one advertisement) is called route summari- zation, as shown in Figure 3.5. In some of the literature, you may find route summarization referred to as route aggregation. FIGURE 3.5 Route summarization Besides reducing the number of routing entries that a router must keep track of, route summarization can also help protect an external router from making multiple changes to its routing table, due to instability within a par- ticular subnet. For example, let’s say that we were working on a router that connected to 172.16.2.0/24. As we were working on the router, we rebooted it several times. If we were not summarizing our routes, an external router would see each time 172.16.2.0/24 went away and came back. Each time, it would have to modify its own routing table. However, if our external router I am the way to get to network 172.16.0.0/16 172.16.2.0/24 172.16.1.0/24 172.16.3.0/24 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Extending IP Addresses 85 were receiving only a summary route (i.e., 172.16.0.0/16), then it wouldn’t have to be concerned with our work on one particular subnet. We will get the most benefit from route summarization when the net- works or subnetworks that we are summarizing are contiguous. To illustrate this point, let’s look at an example. Route Summarization Example 1 We have the following networks that we want to advertise as a single sum- mary route: 172.16.100.0/24 172.16.101.0/24 172.16.102.0/24 172.16.103.0/24 172.16.104.0/24 172.16.105.0/24 172.16.106.0/24 To determine what the summary route would be for these networks, we can follow a simple two-step process. 1. Write out each of the numbers in binary, as shown in Table 3.14. TABLE 3.14 Summary Example IP Network Address Binary Equivalent 172.16.100.0 10101100.0001000.01100100.0 172.16.101.0 10101100.0001000.01100101.0 172.16.102.0 10101100.0001000.01100110.0 172.16.103.0 10101100.0001000.01100111.0 172.16.104.0 10101100.0001000.01101000.0 172.16.105.0 10101100.0001000.01101001.0 172.16.106.0 10101100.0001000.01101010.0 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 86 Chapter 3 IP Addressing 2. Examine the table to determine the maximum number of bits (starting from the left) that all of the addresses have in common (where they stop lining up; we bolded them to make them easier for you to see). The number of common bits is the subnet mask for the summarized address (/20). In our example, we can see from the table that all of the addresses have the first 20 bits in common. The decimal equivalent of these first 20 bits is 172.16.96.0. So, we can write our new summarized address as 172.16.96.0/20. If we were to later add a network 172.16.98.0, it would need to come off the router summarizing this address space. If we didn’t, it could cause problems. Okay, this is confusing, we know. This is why we’re going to give you three more examples. Route Summarization Example 2 In this example, we will summarize 10.1.0.0 through 10.7.0.0. First, put everything into binary, and then follow the bits, starting on the left and stop- ping when the bits do not line up. Notice where we stopped boldfacing the following: Now, create the network number using only the boldfaced bits. Do not count the bits that are not in boldface. The second octet has no bits on (bits in the bolded section), so we get this: 10.0.0.0 10.1.0.0 00001010.00000001.00000000.00000000 10.2.0.0 00001010.00000010.00000000.00000000 10.3.0.0 00001010.00000011.00000000.00000000 10.4.0.0 00001010.00000100.00000000.00000000 10.5.0.0 00001010.00000101.00000000.00000000 10.6.0.0 00001010.00000110.00000000.00000000 10.7.0.0 00001010.00000111.00000000.00000000 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Extending IP Addresses 87 To come up with the summary mask, count all the bolded bits as ones. Because eight bits are boldface in the first octet and five bits in the second, we’ll get this: 255.248.0.0 Route Summarization Example 3 This example will show you how to summarize 172.16.16.0 through 172.16.31.0. First, let’s put the network addresses into binary and then line up the bits. 172.16.16.0 10101100.0001000.00010000.00000000 172.16.17.0 10101100.0001000.00010001.00000000 172.16.18.0 10101100.0001000.00010010.00000000 172.16.19.0 10101100.0001000.00010011.00000000 172.16.20.0 10101100.0001000.00010100.00000000 172.16.21.0 10101100.0001000.00010101.00000000 172.16.22.0 10101100.0001000.00010110.00000000 172.16.23.0 10101100.0001000.00010111.00000000 172.16.24.0 10101100.0001000.00011000.00000000 172.16.25.0 10101100.0001000.00011001.00000000 172.16.26.0 10101100.0001000.00011010.00000000 172.16.27.0 10101100.0001000.00011011.00000000 172.16.28.0 10101100.0001000.00011100.00000000 172.16.29.0 10101100.0001000.00011101.00000000 172.16.30.0 10101100.0001000.00011110.00000000 172.16.31.0 10101100.0001000.00011111.00000000 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 88 Chapter 3 IP Addressing Notice where the bits stop lining up (in boldface). Count only the bits that are on (ones) to get the network address: 172.16.0.0 Now, create the summary mask by counting all the bits that are in bold- face up to the point where they stop lining up. We have eight bits in the first octet, eight bits in the second octet, and four bits in the third octet. That is a /20 or 255.255.240.0 Boy, that sure seems like a pain in the pencil, huh? Try this shortcut. Take the first number and the very last number, and put them into binary: Can you see that we actually came up with the same numbers? It is a lot easier than writing out possibly dozens of addresses. Let’s do another exam- ple, but let’s use our shortcut. Route Summarization Example 4 In this example, we will show you how to summarize 192.168.32.0 through 192.168.63.0. By using only the first network number and the last, we’ll save a lot of time and come up with the same network address and subnet mask: First number: 192.168.32.0 = 11000000.10101000.00100000.00000000 Last number: 192.168.63.0 = 11000000.10101000.00111111.00000000 Network address: 192.168.32.0 Subnet mask: 255.255.224.0 172.16.16.0 10101100.0001000.00010000.00000000 172.16.31.0 10101100.0001000.00011111.00000000 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Extending IP Addresses 89 Design Considerations for Route Summarization Keep the following information in mind when designing your network sum- marization points: Only classless routing protocols support route summarization. Exam- ples of classless routing protocols include RIPv2, EIGRP, and OSPF. Therefore, if you are working in a RIPv1 or IGRP environment, route summarization is not going to work for you. Classless and classful protocols were discussed in Chapter 2, “Routing Principles.” Route summarization is most effective when the addresses have been organized in a hierarchy (i.e., “hierarchical addressing”). When we speak of addresses being hierarchical, we mean that the IP subnets at the “bottom of the tree” (i.e., the ones with the longest subnet masks) are subsets of the subnets at the “top of the tree” (i.e., the ones with the shortest subnet masks). Figure 3.6 will be used to illustrate hierar- chical versus non-hierarchical addressing. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 90 Chapter 3 IP Addressing FIGURE 3.6 Discontiguous networking example In the VLSM section of this chapter, we discussed how route summariza- tion in discontiguous networks could cause some hosts to become unreach- able, as we saw in Figure 3.4. If both RouterA and RouterB are sending out advertisements to the WAN cloud advertising that they are the path to net- work 172.16.0.0/16, then devices in the WAN cloud will not know which advertisement to believe. Hierarchical Adressing 10.1.0.0/16 10.1.2.8/3010.1.2.4/3010.1.1.8/3010.1.1.4/30 10.1.1.0/24 10.1.2.0/24 Non-Hierarchical Adressing 10.1.0.0/16 10.1.2.8/3010.1.2.4/3010.3.0.0/16172.16.2.0/24 172.16.1.0/24 10.2.0.0/16 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Extending IP Addresses 91 Remember that you can avoid this situation by proper address planning ahead of time. However, you may find yourself in a situation where you are dealing with a legacy installation, and you need to overcome this issue of dis- contiguous networks. One solution is to turn off route summarization on the routers. To keep routing protocols such as RIPv2 and EIGRP from automatically summariz- ing routes, we can explicitly disable route summarization in the Cisco IOS. Following are examples of IOS configurations, where we are disabling auto- matic route summarization. As the OSPF chapters will show, OSPF does not automatically summarize. To turn off auto-summarization for RIP version 2 routed networks, use the following router configuration: router rip version 2 network 10.0.0.0 network 172.16.0.0 no auto-summary To turn off auto-summarization for EIGRP routed networks, use the fol- lowing router configuration: router eigrp 100 network 10.0.0.0 network 172.16.0.0 no auto-summary Another way to allow discontiguous networks to be interconnected over a serial link is to use Cisco’s IOS feature called IP unnumbered. We’ll look at this next. IP Unnumbered With IP unnumbered, a serial interface is not on a separate network, as all router interfaces tend to be. Instead, the serial port “borrows” an IP address from another interface. In the following router configuration example, inter- face Serial 0 is using a borrowed IP address from interface Ethernet 0: interface serial 0 ip unnumbered ethernet 0 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 92 Chapter 3 IP Addressing Therefore, by using IP unnumbered, the apparently discontiguous subnets, shown in Figure 3.4, are actually supported. Understand that both sides of the network must be the same address class. In other words, you can’t bor- row an IP address on one side from a 10.0.0.0 network and then from 172.16.0.0 on the other side of the point-to-point link. There are a few things to be aware of before using IP unnumbered interfaces. For example, IP unnumbered is not supported on X.25 or SMDS networks. Also, since the serial interface has no IP number, you will not be able to ping the interface to see if it is up, although you can determine the interface status with SNMP. In addition, IP security options are not supported on an IP unnum- bered interface. Decimal-to-Binary Conversion Chart For your convenience, Table 3.15 provides a decimal-to-binary chart to help you with your IP addressing. TABLE 3.15 Decimal-to-Binary Chart Decimal Binary Decimal Binary Decimal Binary Decimal Binary 0 00000000 16 00010000 32 00100000 48 00110000 1 00000001 17 00010001 33 00100001 49 00110001 2 00000010 18 00010010 34 00100010 50 00110010 3 00000011 19 00010011 35 00100011 51 00110011 4 00000100 20 00010100 36 00100100 52 00110100 5 00000101 21 00010101 37 00100101 53 00110101 6 00000110 22 00010110 38 00100110 54 00110110 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com [...]... Interdomain Routing (CIDR) IP address IP unnumbered octets route summarization subnet mask Variable-Length Subnet Mask (VLSM) Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Written Lab 97 Commands Used in This Chapter Here is the list of commands used in this chapter: Command Description no auto-summary Used to disable the automatic route summarization performed by various classless routing protocols,... effective in which of the follow- ing environments? (Choose all that apply.) A When a large number of contiguous network numbers are being advertised B When IGRP is being used as the routing protocol C When EIGRP is being used as the routing protocol D When VLSMs are in use 19 If you have an IP address of 172.16.1.10/25, what is the broadcast address that the host will use? A 172.16.255.255 B 172.16.1.255... => 14 hosts 255.255.255.0 => 8 host bits => 254 hosts 255.255.255.192 => 6 host bits => 62 hosts 255.255.255.224 => 5 host bits => 30 hosts 4 B, D VLSM is compatible only with classless routing protocols Classless routing protocols have the ability to carry subnet information in their route advertisements RIPv1 and IGRP are classful, whereas RIPv2 and EIGRP are classless 5 A Route summarization, which... octets equal 32 bits Since a byte is eight bits, four bytes also equal 32 bits 18 A, C, D Route summarization is not compatible with IGRP, because IGRP is a classful routing protocol, meaning that it does not carry subnet information in its routing updates 19 D With 25 bits of subnetting, we have the last seven bits to use as the host address If we set each of these last seven bits to 1 (the definition... the following subnet masks will support 50 IP addresses? (Choose all that apply.) A 255.255.255.240 B 255.255.255.0 C 255.255.255.192 D 255.255.255.224 4 VLSM is compatible with which of the following routing protocols? (Choose all that apply.) A RIPv1 B RIPv2 C IGRP D EIGRP Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 106 Chapter 3 IP Addressing 5 Which of the following best describes route... cases, make a discontiguous network appear as contiguous across a serial link Also, if you disable route summarization, then a router will advertise each individual subnet However, answer D only works with routing protocols that can carry subnet information, otherwise you still end up with a discontiguous network, for example EIGRP and RIP V2 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 112 Chapter... chapter discussed how to preserve IP addresses by using VLSMs (Variable-Length Subnet Masks) It also examined various design considerations, such as using contiguous network addressing and using classless routing protocols (e.g., RIPv2 and EIGRP) Next, we introduced the concept of route summarization We saw how router resources, such as memory and processor cycles, could be preserved by representing contiguous . reducing the number of routing entries that a router must keep track of, route summarization can also help protect an external router from making multiple changes to its routing table, due to. designing your network sum- marization points: Only classless routing protocols support route summarization. Exam- ples of classless routing protocols include RIPv2, EIGRP, and OSPF. Therefore,. see each time 172.16.2.0/24 went away and came back. Each time, it would have to modify its own routing table. However, if our external router I am the way to get to network 172.16.0.0/16 172.16.2.0/24 172.16.1.0/24 172.16.3.0/24 Copyright